It has become known to sense the load upon a load support means of an industrial truck in order to determine operative parameters and dangerous situations and to determine the load for the operator. A simple possibility is to measure the pressure of the hydraulic fluid within the lifting cylinder of the lifting device. Various apparatus means such as weighing cells, strain gauges, etc. have become known for this purpose. These means, however, generally are not suited to compensate for temperature and aging dependent effects caused by variations of the viscosity of the hydraulic fluid and the friction of the lifting mechanics and the lifting cylinder. Accordingly, they generally provide for relatively inaccurate measurements.
DE 101 46 421 A1 discloses a method for compensating for variations of the viscosity for a load measuring system. The known method comprises determining a plurality of measuring values for a first load weight and a second load weight. When determining these measuring values also the temperature is measured. When a third load is being lifted, a further plurality of pressure measuring values and also the temperature are determined. The weight of the third load is determined from the first measuring values and the second measuring values, with the temperature being taken into consideration. In other words, the respective load weight is determined by means of a temperature dependent reference curve that had been determined for the first and second loads. This method is more precise than the above mentioned method; however, it does not take into consideration frictional effects. When the load is being lowered, friction forces act in the opposite direction, and the same is true for lifting operations; in both cases friction results in reduction of the pressure within the lifting cylinder.